1. Field of the Invention
The present invention relates to the field of sealing in rotating machines, and more particularly to a leaf seal.
2. Brief Description of the Related Art
A gas turbine includes a rotor, on which various stages with compressor blades and turbine blades are provided, and a stator housing. The rotor is mounted in bearings at each end of the rotor shaft.
The control of the gas flow inside the gas turbine is of paramount importance with regard to both the functionality and the effectiveness. Sealing techniques are used at various locations along the rotor shaft in order to reduce the axial gas flow along the shaft. This is especially important next to the bearings in order to prevent the oil which is used in the bearings from overheating due to the hot gases of the gas flow.
Two types of sealing techniques are traditionally used in this situation—usually alternatively, sometimes also in combination. These are labyrinth seals and brush seals.
Labyrinth seals have no metal-to-metal contact between the rotor and the stator; the sealing effect is therefore relatively small. However, they offer the advantage of low rotational friction and of a therefore virtually unlimited service life.
On the other hand, brush seals have higher friction losses on account of the friction between the bristle ends and the rotor shaft. This results in wear, which limits the service life of the seal. However, brush seals stem the axial gas flow more effectively, in particular in the case of higher axial pressure differences.
The use of these techniques for sealing in gas turbines has numerous restrictions. First, the axial pressure difference that they can withstand is still fairly low. In the case of the brush seals, this is due to the bristles, which have the same stiffness in the axial and circumferential directions: high pressures can cause the bristles to yield in the axial direction. The capability of the seals to allow a significant radial movement and to resist it is also low.
The design of a brush seal is often a compromise between the use of a supporting plate, which is intended to give sufficient axial support, and the non-restriction of the radial movement.
In order to avoid the disadvantages of the known brush seals, a leaf seal has been proposed in U.S. Pat. No. B1-6,343,792, this leaf seal performing the same function as either a labyrinth seal or a brush seal but having the advantages of both. Instead of the bristles, which are produced from wires of circular cross section, thin metal leaves are assembled in a certain arrangement (see, for example, FIG. 3 of U.S. Pat. No. B1-6,343,792 or FIG. 1 of the present application). The leaves, which are oriented with their surfaces essentially parallel to the axial direction, are much stiffer in the axial direction than in the circumferential direction. Thus the seal can withstand higher pressure differences without restricting their possibilities for allowing radial movements. The wide region on the rotor, which is swept by the tips of the leaves, provides the opportunity of producing a hydrodynamic force during operation, and this hydrodynamic force can separate the leaf tips from the shaft. In this way, a distance of a few microns can be produced and maintained, so that the wear, the friction heat and the friction losses can be reduced virtually to zero.
The basic design relates to a number of thin metal leaves which have a controlled air gap between them and are fastened at a predetermined angle to the radius. The air gap is a critical design parameter: it enables an air flow to occur in order thus to produce the hydrodynamic effect; however, it must not be so large as to allow an excessive axial air flow.
Two variants of leaf spring designs are possible: in the one variant, the leaves are blown downward, but in the other they are blown upward. In the variant having the leaves blown downward, there is a distance between the leaf tips and the shaft during the assembly and start-up, and this gap is reduced to very small values by the use of an air flow between the leaves. On the other hand, in the variant having the upward blowing, there is slight mutual influencing between the leaf tips and the shaft during the start-up, and a distance is produced when the shaft is accelerated. In both cases, the flow of the medium through the air gaps between the leaves is critical, as is the control of the seal's inside diameter, which is produced by the leaf tips.
The air flow through the leaves can be varied by using a front and a rear plate which leave a narrow gap free between the surfaces of the leaf stack and the plates (see abovementioned FIGS. 1 and 3). A careful design of these geometries makes it possible to control the upward or downward blowing effects. It may also be desirable to assist the downward blowing effect by an active pressure feed along the length of the leaves or inward from the front side or from the rear directions.
One of the other main advantages of the leaf seal concept is a greater tolerance of the radial movement than in labyrinth or brush seals. This requires a large distance there between the inside diameter of the front and rear end plates and the shaft.
Depending on the geometry selected for the seal and on the diameter of the shaft to be sealed, the number of leaves can be several thousand or many thousand. The accuracy with which said leaves can be produced, assembled and connected, in the course of which a reproducible air gap between each pair of leaves is ensured, is critical for the successful implementation of every possible seal design.
The joining method for fixing the leaves in their position could be a mechanical technique, such as clamping in place, welding, or brazing or any possible combination thereof. It is quite obviously important that a minimum disturbance of the leaves or of their relative positions occurs during the joining process.
Various joining methods have already been proposed in the abovementioned publication U.S. Pat. No. 1-6,343,792. In the exemplary embodiments pertaining to FIGS. 1 to 21 of that publication, the leaves, with their top transverse edge, are brazed in place in a housing. In the exemplary embodiments according to FIGS. 22 to 28, the leaves are fastened in curved segments by a known welding method, such as laser welding, electron beam welding (EBW) or TIG welding, the welding being effected in the radial direction from outside through the segment up to the top transverse edges of the leaves (see FIG. 25 and the description on page 20, lines 7-48). The distance between the leaves can be set in this case by positioning elements (FIGS. 22 to 24) embossed in the leaves, by separate spacers (FIG. 27A), or by integral spacers (FIG. 27B). With regard to the use of the electron beam welding, no further details are given in the publication.
Electron beam welding is a method which is available on an industrial basis for the development or production of devices assembled from components within a wide range of various alloys and geometries.
The nature of the heat input, focussed to a high degree, and the accuracy with which the method can be controlled make it especially suitable for the welding of leaves, with or without spacers, for leaf seals.
Electron beam welding involves the use of special equipment in order to generate the electron beam. This equipment includes a cathode in order to emit electrons, which are then accelerated down an evacuated column by means of high voltage and are focussed on the substrate as a narrow beam with accurately controlled energy and position.
The penetration depth of the beam changes with the beam energy and the density of the target material, but is normally within a range of a few 10 microns right up to a few millimeters. The material volume affected is quickly melted, and a fusion welded joint with the surrounding material is produced.
For the optimum use of electron beam welding, those surfaces of the parts (23a, b in FIG. 5 of the present application) to be connected to one another which are to be welded should be in close contact (joint 24), so that a weld 25 capable of bearing load is obtained. This is different from most welding methods (see FIG. 4 of the present application), in which an intermediate space 21 is normally required between the parts 20a, b to be connected as filling space for a filler material in order to produce a weld 22 capable of bearing load.
The lower values of the disturbance in the material which are to be encountered in electron beam welding and are inherent in the method make the method especially suitable for the welding of thin components such as leaves, which are especially susceptible to such disturbances.
One aspect of present invention proceeds from the application of electron beam welding in the production of leaf seals which are assembled from individual leaves, with or without separate spacers.
In this case, the leaves must be produced from a suitable material which can easily be welded by electron beam welding. The design of the leaves and the way in which they are put together must be carefully controlled in order to optimize the joining by means of electron beam welding.
In particular, other aspects of the present invention start from the fact that successful electron beam welding is based on close contact between the surfaces to be connected in order to minimize the disturbances occurring during the welding. This close contact is not ensured in the joining technique as disclosed in publication US-B1-6,343,792. The welding there is effected in the radial direction from outside through the curved segment-shaped holding element and includes the top transverse edges of the leaves, which, on account of their arrangement in a circle, are relatively far apart. For the abovementioned reasons, such a configuration of the welding process involves considerable disadvantages for the application of electron beam welding.